VINYLLITHIUM - DYNAMIC BEHAVIOR IN TETRAHYDROFURAN SOLUTION AND COMPREHENSIVE ANALYSIS OF NMR SPIN-SPIN COUPLING-CONSTANTS

Vinyllithium (ViLi; isotopically labeled with Li-6) is an 8:1 tetramer/dimer equilibrium in tetrahydrofuran-d8 (THF-d8) at -90-degrees-C (1.6 M solution). In the presence of equilmolar amounts of TMEDA (tetramethylethylenediamine), the tetramer:dimer ratio is 1:13 in THF-d8 at -80-degrees-C (1.9 M solution). Dynamic NMR phenomena are observed in the ViLi tetramer: at -90-degrees-C in THF-d8, a ''static'' aggregate is found, whereas at -60 -degrees-C rapid intraaggregate exchange of the four lithium sites indicates a ''fluxional'' tetramer. Within a ''static'' tetramer, rapid tumbling of a monomer unit is deduced from C, Li and H, Li NMR coupling patterns. From tilts within cross peak multiplets in H-1, C-13 HETCOR spectra of ViLi (due to coupling with the passive Li-6 spin), it is concluded that the sign of 1J(C-13, Li-6) is positive. In a fully coupled heteronuclear Li-6, H-1 shift correlation experiment (FUCOUP), an intense and a weaker cross peak are observed between Li-6, H(gem) and Li-6, H(trans), respectively, of tetrameric ViLi. This is indicative of scalar Li-6, H-1 coupling. In a Li-6, H-1 FUCOUP spectrum with a small (36-degrees) H-1 mixing pulse (''beta-FUCOUP''), the tilt of the Li-6, H(trans) cross peak indicates the sign and magnitude of 2J(Li-6, H(gem)) to be +0.4Hz. Analogous to the tetramer, Li-6, H-1 beta-FUCOUP of the dimer reveals 3J(Li-6, H(trans)) to be +0.4 Hz. The C-13, C-13 coupling constant in vinyllithium is extraordinarily small (dimer, 35.0 Hz; tetramer, 36.3 Hz). Likewise,the one-bond C-13, H-1 coupling constant 1J(C1, H(gem)) is the lowest reported so far for monosubstituted ethylenes (dimer, 91.7 Hz; tetramer, 93.2 Hz). By combination of C-13 gated decoupled and C-13, H-1 beta-FUCOUP data, extreme numbers are also found for geminal 2J(C-13, H-1) coupling constants: dimer, 2J(C1, H(cis)) = +13.5 HZ; 2J(C1, H(trans) = -5.9 Hz; 2J(C2, H(gem)) = -10.5 Hz; tetramer, 2J(C1, H(cis)) = +13.5 HZ; 2J(C1, H(trans)) = -6.1 HZ; 2J(C2,H(gem)) = -12.1 Hz.